1. Field of the Invention
The present invention relates to a noise reduction technique in an image capture apparatus that includes an image sensor.
2. Description of the Related Art
Recently, image sensors mounted on electronic cameras such as digital cameras have tended to reduce the amount of accumulated charge per pixel due to a reduction in size as well as an increase in the number of pixels. In order to maintain the sensitivity at the same level as in conventional image sensors, compensation must be performed by amplifying a signal obtained when reading out accumulated charges. Since noise components are also amplified during signal amplification, random noise becomes larger than before, resulting in a degradation in the S/N ratio. Because the sensitivity is raised in order to widen the range of conditions suitable for image capture, noise reduction techniques are becoming more important than ever.
A method often used in the noise reduction technique for a single captured image is known to perform low-pass filter processing using a two-dimensional filter, as in, for example, Japanese Patent Laid-Open No. 11-041491. The output value of a pixel of interest is obtained by adding and averaging the output values of peripheral pixels for which the absolute value of the difference from the pixel output value of the pixel of interest is less than or equal to a predetermined threshold. This processing allows a reduction in random noise having an amplitude less than or equal to the predetermined threshold. On the other hand, an object edge portion or the like with an amplitude larger than the predetermined threshold remains unchanged. This minimizes degradation in the feeling of resolution. However, the predetermined threshold must be set large to cope with random noise of a relatively large amplitude. This makes the random noise indistinguishable from an object edge with a small amplitude. For this reason, although the noise reduction effect is enhanced, the feeling of resolution is lost. As the number of taps of the filter increases, the number of pixels to be subjected to addition and averaging increases. Hence, although the noise reduction effect is enhanced, circuit scale is increased.
To solve the above-described problems, for example, Japanese Patent Laid-Open No. 2007-329905 proposes an arrangement for performing noise reduction processing using image data read out from an image sensor in two image reading modes, that is, low and high resolution modes.
Japanese Patent Laid-Open No. 2001-057677 proposes an arrangement which performs noise reduction processing by reducing an original image to generate a reduced image having a resolution lower than that of the original image, enlarging the reduced image to the size of the original image, and then compositing the enlarged image with the original image.
These techniques make it possible to attain a high random noise reduction effect and a high-resolution still image with a simple arrangement.
In Japanese Patent Laid-Open No. 2007-329905, switching between the two driving modes is performed. When capturing an image of a moving object, it is difficult to acquire images of the same object at the same angle of view in the two driving modes.
In Japanese Patent Laid-Open No. 2001-057677, to execute the above-described composition processing directly using RGB signals generated by the image sensor, it is necessary to generate three RGB color planes to generate a reduced image of arbitrary size. To do this, memory usage is greatly increased during the noise reduction processing. In order to prevent such an increase in memory usage, the composite processing is performed after converting RGB signals into YUV signals. Conversion to YUV signals enables the occupied area of the memory to be made smaller than in the arrangement for generating three RGB color planes.
However, if, for example, the YUV signals after noise reduction processing have the YUV 422 data structure, they include two luminance signals (Y) and two chrominance signals (U, V), that is, a total of four signals for two pixels. For this reason, if the number of bits of the RGB signals generated by the image sensor equals that of the YUV signals, the occupied area of the memory is doubled. That is, the memory area occupied for composition processing is increased.